Plastic containers, which are typically rotationally molded, injection molded or blow molded, usually require a connector at the container's outlet. The connection between the connector and the outlet must be sealed to prevent leakage. One common approach is to drill a hole through the side wall of the tank and attach a bulkhead fitting to the opening. The fitting includes inner and outer seal plates which engage the inner and outer walls around the opening. The bulkhead is secured by tightening the plates. A connector is then threaded into or onto the bulkhead fitting. It is difficult to maintain the seal between the fitting and the wall of the container, however. Since plastic flows in response to stress (cold flow), gaps can form around the bulkhead fitting, causing leakage. In addition, due to the space taken up by the plates, the outlet must be positioned above the bottom of the container, preventing complete drainage. The bulkhead can also be heavy, expensive, and difficult to attach. This increases the cost of the container as well as the number of containers rejected due to improper fitting.
In addition, bulkhead fittings sometimes require the use of an additional gasket to assist in tightening and sealing the plates. Such gaskets can be chemically incompatible with the contents of the container and can cause slippage when compressed.
Instead of a bulkhead fitting, an internally threaded outlet for receiving a connector can be formed in the container during the molding process. A connector, in the form of a short length of pipe having external threads, can then be screwed into the outlet. A valve with internal threads can be screwed onto the protruding end of the connector. The engagement between the threads of the connector and the threads of the outlet forms a better seal than does a bulkhead fitting, due to the increased sealing surface area. This seal, however, is still subject to leakage due to the natural cold flow of the plastic, stress fractures from use, high pressure or hard to seal fluids.
An improved molded outlet for plastic containers is needed to prevent leakage around connectors and valves simply and inexpensively.